The development of art, culture, and advanced cognitive ability that define modern humans may not have evolved until 50,000 years ago, according to a new study published online Monday in the Proceedings of the National Academy of Sciences.

Richard Klein of Stanford University, the senior author of the study, believes that modern humans evolved at the same time that they left Africa to populate Eurasia and the rest of the world.

According to his theory, the cultural innovations led to an increase in fitness for the fledgling human species, spurring a major population expansion.

A competing theory is that humans evolved into their modern form earlier and their population increased gradually until they outstripped their resources around 50,000 years ago, when they indisputably left Africa. Proponents of this theory cite the appearance of proto-jewelry and art in South Africa 85,000 to 65,000 years ago as evidence that humans were already fully modern by that time, with their own system of communication and rudimentary culture.

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Klein decided to test the “modern humans early” vs. “modern humans late” hypothesis by examining ancient population size, with the expectation that the emergence of modern humans should be correlated with a boom in population.

Estimating populations from so long ago is no easy matter, but Klein and other zooarchaeologists have found an unorthodox way of doing so — examining the remains of the animals humans ate.

“Food is the basis of everything; you have to eat to survive,” said Teresa Steele, a zooarchaeologist and paleoanthropologist at UC Davis, who worked on the study with Klein. “We can use things like animal bones and mollusks [shells] to reconstruct ancient subsistence practices.”

In the late 1970s, Klein began comparing the sizes of different shellfish found in archaeological deposits in South Africa. The shellfish were part of early humans’ diet, found by the thousand in ancient trash heaps along with the bones of other animals such as seals, antelopes, and zebras.

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He noticed a general pattern — the younger sites contained smaller shells. After finding similar results in Hawaii, Papua New Guinea, and California, zooarchaeologists concluded that the smaller shells were a sign of larger human populations that had depleted the larger animals and moved on to smaller prey.

For the National Academy of Sciences study, Klein and Steele measured the size of thousands of limpet and turban shells -- two of the most commonly hunted shellfish groups -- from archaeological sites on the south and west coasts of South Africa. Evidence of early art and jewelry, such as shells with holes and fragments of ocher rock (possibly used for pigment), had been recovered in the same area, suggesting the emergence of modern human behavior.

“The implication would be that people were already behaving in a modern way, making jewelry and art 75,000 years ago, which is 25,000 years before the out-of-Africa expansion,” Klein said.

Klein and Steele found what they expected — shells were generally smaller in Late Stone Age deposits (about 1,000 to 12,000 years old) than in Middle Stone Age deposits (about 60,000 to 120,000 years old), reflecting growth in the human population over that time.

But shell size remained constant during the Middle Stone Age, implying that the early human populations also remained constant, even as they developed the earliest forms of jewelry and art.

The finding supported Klein’s hypothesis that humans did not become fully modern until their population expansion that accompanied the African exodus 50,000 years ago.

Klein thinks the finding also casts doubt on the significance of the early jewelry and art. Given that few pieces have been found — one in tens of thousands of ocher rocks had any significant patterning — perhaps the objects have been mistaken as art.

“If you look at the jewelry, it’s not very compelling, not very impressive,” he said. “There are tiny pieces of shells about the size of your thumbnail that have holes in them. The holes are not clearly made by tools, but they could be beads. Maybe they are, maybe they aren’t .… Whether these things are art or jewelry really is in the eye of the beholder.”

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Klein has some trouble believing that the artifacts represent any great innovation symbolic of modern human society. If these artifacts really were the hallmark of a society with art and culture, and the people in South Africa were modern humans by 75,000 years ago, why didn’t they expand to Europe then? Why did they wait another 25,000 years?

One caveat of the study is that there were no shellfish examined from 50,000 years ago, when humans left Africa; the closest date was 60,000 years ago. But the prospects for finding an archaeological site in this time window are grim, as coastal sites from 60,000 to 12,000 years ago are now underwater thanks to sea level changes.

In the meantime, Klein and his fellow zooarchaeologists will keep combing beaches for shells -- clues to our evolutionary past.

[For the record 1:08 p.m. PDT July 18, 2013: An earlier version of this post misspelled Teresa Steele’s first name as Theresa.]